New therapeutics that target pathways essential for tumor angiogenesis are of significant interest in the treatment of tumor growth and metastasis. FDA-approved antiangiogenic agents such as bevacizumab that inhibit VEGF signaling have yielded varying results. While anti-VEGF therapies can be quite effective at suppressing tumor vasculatures, recent reports suggest that long-term treatment may yield serious toxicity or result in development of resistance. Endoglin is a transforming growth factor- (TGF-) co-receptor that is emerging as a unique target in antiangiogenic therapy. Endoglin is: 1) required for both normal and tumor- induced angiogenesis; 2) the gold standard biomarker of tumor vasculatures; and 3) strongly correlated with tumor progression, survival rate, and metastases. TRC105 is a humanized endoglin monoclonal antibody (mAb) currently in phase I/II trials for treatment of advanced solid or metastatic cancer. While early reports indicate that TRC105 improves tumor response and has a safety mechanism distinct from VEGF inhibitors, its long-term efficacy (i.e. sensitivity, resistance, and side effects) remains to be determined. Moreover, despite recent advances, the fundamental mechanism by which TRC105 and related endoglin mAbs inhibit tumor vasculatures is poorly understood. Our preliminary studies reveal unique antiangiogenic properties of TRC105 that are independent of cell-growth inhibition or apoptosis. Instead, TRC105 critically alters the critical balance of TGF- signaling to the Smad pathways, impairs endothelial migration, and promotes endoglin shedding to produce a circulating antiangiogenic factor (soluble endoglin). Based on our initial findings, we will investigate the underlying mechanisms, evaluate other endoglin-targeting mAbs for distinct epitope-related effects, and test for enhanced efficacy in combination therapy with bevacizumab. We will employ multidisciplinary approaches including TGF- and VEGF signaling PCR arrays to identify novel drug-sensitive targets and biomarkers, innovative biophysical studies to characterize important Ab-induced ligand binding characteristics, and validate the key mechanisms in vivo. Together, these studies will provide the first molecular and cellular data for endoglin-targeting therapies, identify new pharmacological targets for intervention, and help develop new strategies for antiangiogenic therapies.